According to a diagnosis apparatus disclosed in U.S. Pat. No. 6,918,245 B2 (JP-A-2004-11585), a valve is operated to communicate and block a secondary air supply passage when an air pump is operated to supply secondary air into an exhaust passage through the secondary air supply passage. In this diagnosis apparatus, differential pressure between the upstream of the valve and the downstream of the valve in the secondary air supply system is detected, so that it is evaluated whether an abnormity arises in the secondary air supply system, in accordance with the differential pressure.
Pressure in the secondary air supply apparatus changes due to variation in the atmospheric pressure. In addition, an amount of secondary air supplied from the air pump changes due to variation in voltage of a battery, which supplies electricity to the air pump. Furthermore, when a load applied to the air pump becomes high, temperature of the air pump increases. In this case, capacity of the air pump decreases, and the flow amount of secondary air decreases. Load applied to the air pump is increased mainly due to increase in exhaust pressure, which is apt to be increased when the vehicle is driven. Accordingly, when the diagnosis operation is performed in a driving condition of the vehicle, increase in exhaust pressure may exert influence to the capacity of the air pump. Furthermore, when the vehicle has a charging device such as a turbocharger, exhaust pressure is apt to be further increased, consequently, the capacity of the air pump is apt to further vary.
For example, pressure is detected in the diagnosis operation, in which the valve is opened, on a valve open timing. Pressure is also detected in the diagnosis operation, in which the valve is closed, on a valve close timing. When it takes a long period between the valve open timing and the valve close timing, pressure detected in the diagnosis operation may be fluctuated in this period. Specifically, the pressure detected in the diagnosis operation may be fluctuated due to variation in parameters, which indirectly exert influence to the diagnosis operation, such as the atmospheric pressure, temperature of the air pump, exhaust pressure, voltage of the battery, and the like. As a result, accuracy of the diagnosis operation may be degraded.
According to JP-A-7-119451, a diagnosis apparatus changes a threshold for evaluating an abnormity in a secondary air supply system, in accordance with an amount of intake air, or the like. When the amount of intake air changes, an amount of secondary air supplied to the exhaust pipe changes. In this operation, the diagnosis apparatus is capable of properly setting the threshold, so that accuracy of the detecting operation can be enhanced.
However, as temperature of an air pump in the secondary air supply system increases, the capacity of the air pump decreases. When the temperature of the air pump becomes high, the capacity of the air pump decreases. Consequently, the threshold may not properly set by the diagnosis apparatus.
In addition, as load applied to the air pump becomes high, the temperature of the air pump increases. The load applied to the air pump increases mainly due to increase in exhaust pressure. When the diagnosis operation is performed in a condition where the vehicle moves, the exhaust pressure is apt to increase. Consequently, the capacity of the air pump is apt to fluctuate. Furthermore, when the vehicle has a charging device, the exhaust pressure is apt to further increase. Accordingly, the capacity of the air pump is apt to further fluctuate. Variation in temperature of the air pump exerts an unignorable influence to the diagnosis operation.
In general, an exhaust gas purifying apparatus such as a catalyst system is provided to an intake pipe of an internal combustion engine for purifying exhaust gas. Secondary air is supplied to upstream of the purifying apparatus in order to enhance efficiency of the purifying operation of the purifying apparatus. Specifically, a secondary air supply apparatus is provided with a secondary air pipe, an air pump, and a valve. The secondary air pipe connects with the engine. The air pump and the valve are provided in the secondary air pipe. The air pump is operated, and the valve is opened when the engine is started, for example, so that secondary air is supplied into an exhaust pipe through the secondary air pipe.
However, efficiency of purifying exhaust gas may decrease when one of the air pump and the valve cause a disorder in the secondary air supply apparatus. In this condition, engine emission may increase. According to U.S. Pat. No. 6,945,035 B2 (JP-A-2003-201834), when an abnormality is found in a failure diagnosis under a cold environment, and the abnormality is conceived to be caused due to freeze arising in a secondary air supply system, another failure diagnosis is performed after warming up. In this operation, it can be evaluated whether the abnormality is temporally caused due to the freeze arising in the secondary air supply system. In U.S. Pat. No. 6,945,035 B2, the following method is disclosed for evaluating the freeze arising in the secondary air supply system. That is, a valve is opened, and an air pump is operated when an engine is started, so that secondary air is supplied into an exhaust pipe through a secondary air pipe. Subsequently, pressure in the secondary air pipe is detected using a pressure sensor, so that it is evaluated whether components such as the air pump and the valve of the secondary air supply apparatus is frozen in accordance with the detection signal of the pressure sensor.
However, in this operation, the detection signal of the pressure sensor needs to be monitored while secondary air is supplied during the evaluation of the freezing condition. Accordingly, the evaluation of the freezing condition may take long. In addition, the air pump may be operated in a condition where the air passage is blocked. In this operation, excessive load may be applied to the air pump, consequently, the air pump may cause a disorder.